Backplane footprint for high speed, high density electrical connectors

ABSTRACT

A printed circuit board includes a plurality of layers including conductive layers separated by dielectric layers; and at least one via configured for solder attachment to a connector lead of a surface mount connector, the at least one via including a conductive element that extends from an upper surface of the printed circuit board through one or more of the plurality of layers, the conductive element having a recess in a surface thereof. The recess is configured to receive a tip portion of the connector lead of the surface mount connector. The printed circuit board may have via patterns including signal vias and ground vias.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.16/435,781, filed Jun. 10, 2019, which claims priority based onProvisional Application No. 62/683,146, filed Jun. 11, 2018, which ishereby incorporated by reference in its entirety.

BACKGROUND

This patent application relates generally to interconnection systems,such as those including electrical connectors, used to interconnectelectronic assemblies.

Electrical connectors are used in many electronic systems. It isgenerally easier and more cost effective to manufacture a system asseparate electronic assemblies, such as printed circuit boards (“PCBs”),which may be joined together with electrical connectors. A knownarrangement for joining several printed circuit boards is to have oneprinted circuit board serve as a backplane. Other printed circuitboards, called “daughter boards” or “daughter cards,” may be connectedthrough the backplane.

A known backplane has the form of a printed circuit board onto whichmany connectors may be mounted. Conductive traces in the backplane maybe electrically connected to signal conductors in the connectors so thatsignals may be routed between the connectors. Daughter cards may alsohave connectors mounted thereon. The connectors mounted on a daughtercard may be plugged into the connectors mounted on the backplane. Inthis way, signals may be routed among the daughter cards through thebackplane. The daughter cards may plug into the backplane at a rightangle. The connectors used for these applications may therefore includea right angle bend and are often called “right angle connectors.” Otherknown connectors include, but are not limited to, orthogonal midplaneconnectors and midplaneless direct attachment orthogonal connectors.

Connectors may also be used in other configurations for interconnectingprinted circuit boards and for interconnecting other types of devices,such as cables, to printed circuit boards. Sometimes, one or moresmaller printed circuit boards may be connected to another largerprinted circuit board. In such a configuration, the larger printedcircuit board may be called a “mother board” and the printed circuitboards connected to it may be called daughter boards. Also, boards ofthe same size or similar sizes may sometimes be aligned in parallel.Connectors used in these applications are often called “stackingconnectors” or “mezzanine connectors.”

A “midplane” configuration is also sometimes used. The midplane issimilar to a backplane in that it has connectors positioned to makeconnection with multiple daughter boards. In a midplane, however, theconnectors are attached to opposing surfaces of the midplane such thatdaughter boards may be installed in the system on both sides of themidplane. Frequently, the daughter boards on one side are orthogonal tothe daughter boards installed on the other side, which can shorten thesignal paths from one daughter board to another relative to having thedaughter boards aligned in parallel and connected through a backplane.Specialized connectors are sometimes used for a midplane configuration,but that configuration may also be created with the same connectors usedin a backplane configuration.

An orthogonal arrangement of daughter boards may also be created withouta midplane by using connectors on the daughter boards on one side of anelectronic assembly that can directly mate to connectors on the daughterboards on the other side of the assembly. This configuration is called“direct mate orthogonal.” This configuration may be created with rightangle connectors as might be used on a daughter board in a backplaneconfiguration on the daughter boards on one side of the assembly.Connectors designed for this direct mate orthogonal configuration areused on the boards on the other side of the assembly.

Regardless of the exact application, electrical connector designs mustkeep pace with trends in the electronics industry. Electronic systemsgenerally have gotten smaller, faster, and functionally more complex.Because of these changes, the number of circuits in a given area of anelectronic system, along with the frequencies at which the circuitsoperate, have increased significantly in recent years. Current systemspass more data between printed circuit boards and require electricalconnectors that are electrically capable of handling more data at higherspeeds than connectors of even a few years ago.

In a high density, high speed connector, electrical conductors may be soclose to each other that there may be electrical interference betweenadjacent signal conductors. To reduce interference, and to otherwiseprovide desirable electrical properties, shield members are often placedbetween or around adjacent signal conductors. The shields may preventsignals carried on one conductor from creating “crosstalk” on anotherconductor. The shield may also impact the impedance of each conductor,which may further affect electrical properties.

Other techniques may be used to control the performance of a connector.For example, transmitting signals differentially may reduce crosstalk.Differential signals are carried on a pair of conductive paths, called a“differential pair.” The voltage difference between the conductive pathsrepresents the signal. In general, a differential pair is designed withpreferential coupling between the conductive paths of the pair. Forexample, the two conductive paths of a differential pair may be arrangedto run closer to each other than to adjacent signal paths in theconnector. No shielding is desired between the conductive paths of thepair, but shielding may be used between differential pairs. Electricalconnectors can be designed for differential signals as well as forsingle-ended signals.

In an interconnection system, such connectors are attached to printedcircuit boards, one of which may serve as a backplane for routingsignals between the electrical connectors and for providing referenceplanes to which reference conductors in the connectors may be grounded.Typically the backplane is formed as a multi-layer assembly manufacturedfrom stacks of dielectric sheets, sometimes called “prepreg”. Some orall of the dielectric sheets may have a conductive film on one or bothsurfaces. Some of the conductive films may be patterned, usinglithographic or laser printing techniques, to form conductive tracesthat are used to make interconnections between circuit boards, circuitsand/or circuit elements. Others of the conductive films may be leftsubstantially intact and may act as ground planes or power planes thatsupply the reference potentials. The dielectric sheets may be formedinto an integral board structure such as by pressing the stackeddielectric sheets together under pressure.

As in the case of the connectors that attach to the printed circuitboards, the electrical performance of printed circuit boards is at leastpartially dependent on the structures of the conductive traces, groundplanes and vias formed in the printed circuit boards. Further,electrical performance issues become more acute as the density of signalconductors and the operating frequencies of the connectors increase.

To make electrical connections to the conductive traces or ground/powerplanes, holes may be drilled through the printed circuit board. Theseholes, or “vias”, are filled or plated with metal such that a via iselectrically connected to one or more of the conductive traces or planesthrough which it passes.

In one approach for attaching connectors to the printed circuit board,known as “press fit technology”, contact pins or contact “tails” fromthe connectors may be inserted into the vias, with or without usingsolder. The vias are sized to accept the contact tails of the connector.In another approach for attaching connectors to the printed circuitboard, known as “surface mount technology”, the vias are provided withconductive pads on the surface of the printed circuit board and thecontact pins of the connector are soldered to the conductive pads. Inthe surface mount technology, the vias may have smaller diameters thanvias for receiving contact pins of the connector.

As indicated above, a trend in electronic systems is toward increasingminiaturization, higher speed operation and functional complexity. Inorder to meet the demands for miniaturization, the density of connectorpins must increase while avoiding degradation in electrical performance.Likewise, the printed circuit boards to which the high speed highdensity connectors are attached must be miniaturized, while maintainingelectrical performance. In addition, the connectors, the printed circuitboards and the connections between the connectors and the printedcircuit boards must have high reliability.

Press fit connectors have contact tails which are inserted into holes incorresponding vias in the printed circuit board. The contact tails maybe slightly deformed as they are inserted into the via holes, therebyproviding a reliable electrical and mechanical connection. Furthermore,the contact tails may be soldered to the printed circuit board. However,the miniaturization of press fit connectors is limited by the design ofthe press fit contact tails. In other words, the press fit contact tailshave a minimum width to ensure reliable operation. In addition, printedcircuit boards designed for attachment to press fit connectors require azone in the upper portion of the printed circuit board in which the viasare large enough to accept the press fit contact tails, leaving littleor no area for routing signal traces.

Surface mount connectors do not require holes for receiving the contacttails. Instead, the contact pins of the surface mount connectorphysically contact conductive pads on the surface of the printed circuitboard. The contact pins of the connector are soldered to the conductivepads to provide an electrical connection. While surface mount connectorsdo not have the disadvantage of requiring via holes large enough toaccept contact tails, surface mount connectors may have issues withrespect to reliable electrical contact between the contact pin and theconductive pad on the printed circuit board. In particular, the surfacemount contact pins may not be coplanar due to manufacturing tolerances.Furthermore, the conductive pads on the printed circuit board may not becoplanar due to manufacturing tolerances. As a result, when the surfacemount connector is placed in contact with the printed circuit board,some of the surface mount contact pins may contact the conductive padswhile others are spaced above the conductive pads. In the solderingoperation, solder may bridge the gaps between the contact pins and theconductive pads, but in some cases an open connection may result. Evenif the solder does bridge the gap between the contact pin and theconductive pad, the connection may be unreliable.

Accordingly, there is a need for improved printed circuit boards andconnector footprints for high speed, high density electricalapplications.

SUMMARY

The inventors have recognized that interconnection systems require newstructures in order to satisfy requirements for smaller sizes, higheroperating frequencies and increased complexity. An interconnectionsystem in accordance with embodiments includes a surface mount connectorand a printed circuit board having structures for solder attachment toconnector leads of the surface mount connector. In embodiments, theprinted circuit board includes one or more conductive elements on itsupper surface for solder attachment to respective connector leads of thesurface mount connector. The conductive elements may comprise vias thatextend through one or more layers of the printed circuit board.

Each of the conductive elements is provided with a recess that receivesa tip portion of the connector lead. The tip portions of the connectorleads of the surface mount connector may be tapered or otherwiseconfigured for insertion into the recess in the conductive element ofthe printed circuit board. The recess enables a more reliable solderconnection between the conductive element and the connector lead of thesurface mount connector, as compared with a conductive element having aflat surface. The recess mitigates reliability issues caused by lack ofcoplanarity of the connector leads. The interconnection system enables areduction in area requirements as compared with interconnection systemsthat rely upon press fit insertion of connector pins into correspondingvias on the printed circuit board.

In accordance with embodiments, a printed circuit board comprises atleast one dielectric layer and at least one conductive element formed onthe dielectric layer and configured for solder attachment to a connectorlead of a surface mount connector, the conductive element having arecess in a surface thereof.

In accordance with further embodiments, a printed circuit boardcomprises a plurality of layers including conductive layers separated bydielectric layers and at least one via configured for solder attachmentto a connector lead of a surface mount connector, the at least one viacomprising a conductive element that extends from an upper surface ofthe printed circuit board through one or more of the plurality oflayers, the conductive element having a recess in a surface thereof.

In accordance with further embodiments, a printed circuit boardcomprises a plurality of layers including conductive layers separated bydielectric layers and via patterns formed in the plurality of layers,each of the via patterns comprising first and second signal viasextending from a first surface of the printed circuit board to abreakout layer of the conductive layers, and ground vias connected toone or more of the conductive layers, wherein the signal vias and theground vias each include a conductive element having a recess in asurface thereof.

In some embodiments, the recess has a depth in a range of 0.05 mm to0.30 mm.

In some embodiments, the recess has an aspect ratio of depth to width ina range of 1 to 4.

In some embodiments, the via extends from the upper surface of theprinted circuit board to a breakout layer of the conductive layers.

In some embodiments, the recess is centered on the via.

In some embodiments, the recess is offset from a center of the via.

In some embodiments, the at least one via is backdrilled from a secondsurface of the printed circuit board.

In some embodiments, the recess includes a conical portion.

In some embodiments, the recess includes a truncated conical portion.

In some embodiments, the recess includes a cylindrical portion.

In some embodiments, the recess includes a hemispherical portion.

In some embodiments, the via extends from the upper surface of theprinted circuit board to a ground layer of the conductive layers.

In some embodiments, the recess is configured to receive a tip portionof the connector lead of the surface mount connector.

In some embodiments, the recess is configured to receive solder of asolder joint between the at least one via and the connector lead of thesurface mount connector.

In some embodiments, the recess is configured to allow a tip portion ofthe connector lead of the surface mount connector to extend below theupper surface of the printed circuit board.

In some embodiments, the at least one via includes a conductive pad onthe upper surface of the printed circuit board and wherein theconductive pad has a larger diameter than the conductive element of theat least one via.

In some embodiments, the at least one via comprises a cylindricalconductor that extends at least part way through the printed circuitboard.

In some embodiments, the at least one via is filled with a conductivematerial, except for the recess.

In some embodiments, the at least one via is configured for attachmentto a superelastic connector lead of the surface mount connector.

In accordance with further embodiments, a method is provided formanufacturing a printed circuit board comprising a plurality ofconductive layers separated by dielectric layers. The method comprisesdrilling a hole through the printed circuit board, filling the hole withconductive material to form a via, and forming a recess in theconductive material of the via at an upper surface of the printedcircuit board.

In accordance with further embodiments, an interconnection systemcomprises a surface mount component comprising surface mount leads; anda printed circuit board comprising conductive layers separated bydielectric layers, and vias configured for solder attachment torespective leads of the surface mount component, each of the viasincluding a conductive element having a recess in an upper surfacethereof.

In some embodiments, at least 95% of the surface mount leads comprisedistal tips extending into respective recesses.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the disclosed technology, reference ismade to the accompanying drawings, which are incorporated herein byreference and in which:

FIGS. 1 and 2 are cross-sectional views of an interconnection systemincluding a surface mount electrical connector and a printed circuitboard in accordance with embodiments;

FIG. 3 is a cross-sectional view of a printed circuit board, showing avia in accordance with embodiments;

FIG. 4 is a cross-sectional view of a via having a truncated conicalrecess in accordance with embodiments;

FIG. 5 is a cross-sectional view of a via having a cylindrical recess inaccordance with embodiments;

FIG. 6 is a cross-sectional view of a via having a recess with ahemispherical portion and a cylindrical portion in accordance withembodiments;

FIGS. 7A-7C are cross sections of a printed circuit board whichillustrate solder joints between connector leads of a surface mountconnector and a printed circuit board in accordance with embodiments;

FIGS. 8A-8C are cross sections of a printed circuit board whichillustrate solder joints between connector leads of a surface mountconnector and a printed circuit board in accordance with the prior art;and

FIG. 9 is a top view of a printed circuit board showing via patterns inaccordance with embodiments.

DETAILED DESCRIPTION

The inventors have recognized and appreciated that, though substantialfocus has been placed on providing improved electrical connectors inorder to improve the performance of interconnection systems, at somevery high frequencies significant performance improvement may beachieved by inventive designs for printed circuit boards. In accordancewith some embodiments, improvements may be achieved by inventive designsfor connector footprints and for the details of the connectorfootprints, including but not limited to vias of the connectorfootprints. The inventive designs may improve the electrical performanceof the interconnection system, may improve the reliability of theconnection between the printed circuit board and the connector and/ormay facilitate further miniaturization of the interconnection system,for example.

As used herein, the terms “connector footprint” and “backplanefootprint” refer to elements of a printed circuit board which areconfigured to interface to a connector. The connector footprint mayinclude multiple via patterns, and each via pattern may include one ormore signal vias and one or more ground vias. Each via pattern mayfurther include one or more additional vias configured for shielding,impedance control and the like. However, the via patterns are notlimited with respect to the numbers or types of vias. Further, the terms“printed circuit board” and “backplane” are used interchangeably hereinto refer to a structure including conductive layers and dielectriclayers in an alternating arrangement. However, it should be appreciatedthat attachment techniques as described herein may be used to attachconnectors, or any other type of electrical component, to a substratehaving layers and connections between the layers, including daughterboards or midplanes. These techniques may be used with substrates of anythickness.

In some embodiments, the printed circuit board may include a singledielectric layer and a single conductive layer. However, the techniquesdescribed herein may provide valuable advantages in terms of speedand/or manufacturing cost when used with multi-layer boards, such asboards with more than 10 conductive layers carrying signal traces, ormore than 20 layers or in some embodiments, more than 40 layers.Accordingly, in some embodiments, techniques as described herein may beapplied with printed circuit boards with between 10 and 50 layers withsignal traces.

FIGS. 1 and 2 illustrate an example of an electrical interconnectionsystem that may be used in an electronic system, in accordance withembodiments. The electrical connection system includes a connector thatmay be electrically and mechanically connected to a printed circuitboard. In this example, a surface mount connector 100 is configured tobe attached to a printed circuit board 110.

The surface mount connector 100 may include a number of pairs of signalleads and corresponding ground leads. One pair of signal leads andcorresponding ground leads are shown in FIGS. 1 and 2 . Surface mountconnector 100 includes signal leads 120 and 122 and corresponding groundleads 130 and 132. The signal leads 120 and 122 may form a differentialpair. Additional ground leads, not shown, may correspond to each pair ofsignal leads.

As shown in FIGS. 1 and 2 , signal leads 120 and 122 of the surfacemount connector 100 may have beveled tip portions and ground leads 130and 132 may have tapered tip portions. In addition, ground leads 130 and132 may have bent tip portions. As discussed below, these featuresenable insertion of the tip portions into the recesses in respectivevias of printed circuit board 110. In general, the tip portions of theconnector leads of surface mount connector 100 are sized and shaped forinsertion into corresponding recesses in the printed circuit board 110.The tip portions may be beveled, tapered, reduced in dimension orotherwise configured for insertion into corresponding recesses. In somecases, modification of the tip portions of the connector leads may notbe required, and the tip portion may be cylindrical, rectangular, orhave any other suitable shape.

The printed circuit board 110 may include a plurality of conductivelayers separated by dielectric layers, as described below. For example,the printed circuit board 110 may include conductive layers 136separated by dielectric layers 138 in an alternating arrangement. Theprinted circuit board 110 may include a signal via 140 corresponding tosignal lead 120 and a signal via 142 corresponding to signal lead 122.The printed circuit board 110 may further include a ground via 150corresponding to ground lead 130 and a ground via 152 corresponding toground lead 132. The signal leads 120 and 122 are electrically andmechanically connected to the respective signal vias 140 and 142 bysolder joints 180 (shown in FIG. 2 ), and the ground leads 130 and 132are electrically and mechanically connected to the respective groundvias 150 and 152 by solder joints 182 (shown in FIG. 2 ).

As best shown in FIG. 2 , the conductive layer 136 on the top surface ofprinted circuit board 110 may be patterned to form an antipad 144. Theantipad 144 is a region where conductive layer 136 has been removed. Theantipad 144 provides clearance between signal vias 140 and 142 andconductive layer 136, which may be connected to ground. Antipads may beprovided as needed in additional conductive layers of the printedcircuit board 110. Ground vias 150 and 152 are connected to conductivelayers 136, and signal vias 140 and 142 are isolated from conductivelayers 136. Ground vias 150 and 152 may extend through all the layers ofprinted circuit board 110 and may be connected to several conductivelayers 136 which serve as ground planes.

Signal vias 140 and 142 may include conductive pads 146 and 148,respectively, on a top surface 160 of printed circuit board 110. Signalvias 140 and 142 extend from top surface 160 of printed circuit board110 to a breakout layer (shown in FIG. 3 ), where the signal vias 140and 142 are connected to signal traces (shown in FIG. 3 ) forinterconnection to other elements of the printed circuit board. In theembodiment of FIGS. 1 and 2 , signal vias 140 and 142 are backdrilledfrom a bottom surface 162 of printed circuit board 110 to a pointslightly below the breakout layer. Thus, holes 154 and 156 below signalvias 140 and 142, respectively, are air holes.

As shown in FIG. 1 , mounting ends of signal leads 120 and 122 pressagainst conductive structures, here conductive pads 146 and 148, on thetop surface 160 of printed circuit board 110. As a result, edges of thesignal leads 120 and 122 may make contact with signal vias 140 and 142,without requiring the signal vias 140 and 142 to be large enough toaccommodate a pressfit or similar structure. As a result, signal vias140 and 142 may be smaller diameter than in a conventional connectorfootprint, such that the footprint for connector may be smaller diameterthan for a conventional connector. As shown in FIG. 1 , signal vias 140and 142 are filled vias, rather than plated through holes, whichconventionally require a larger diameter than filled vias.

As shown in FIG. 1 , each of the signal vias 140, 142 is provided with arecess 170, and each of the ground vias 150, 152 is provided with arecess 172. In the embodiment of FIGS. 1 and 2 , each of the recesses170, 172 has a truncated conical shape. The recesses are described indetail below.

As shown in FIG. 2 , the recesses 170 and 172 are filled with solder toform solder joints 180 and 182, respectively. The signal leads 120 and122 are electrically and mechanically connected to the respective signalvias 140 and 142 by solder joints 180, and the ground leads 130 and 132are electrically and mechanically connected to the respective conductiveelements 150 and 152 by solder joints 182.

As shown in FIG. 2 , the solder joints 180 and 182 extend into therespective recesses 170 and 172 and improve the reliability of theconnection between the connector leads and the vias of the printedcircuit board. In particular, the recesses 170 and 172 provide a greatersurface area for contact with the solder joints 180 and 182, as comparedwith vias having flat upper surfaces. Furthermore, the recesses 170 and172 permit a greater volume of solder to be utilized in each solderjoint, as compared with vias having flat upper surfaces. The greatersolder volume also improves the reliability of the solder joint. Inaddition, the fact that signal leads 120 and 122 and ground leads 130and 132 may extend into the respective recesses 170 and 172 mitigatesthe effects of lack of coplanarity of the connector leads of the surfacemount connector, as discussed below.

The features of the solder joints 180 and 182, including increasedsolder volume, increased solder contact area and the extension of thesolder joints 180 and 182 into the respective recesses, increase theshear resistance of the solder joints 180 and 182. In particular, thesolder joints 180 and 182 are unlikely to be sheared off in a directionparallel to the surface of the printed circuit board. It will beunderstood that the above features of the solder joints 180 and 182 arenot requirements.

The above-described features of the solder joints 180 and 182, includingincreased solder volume, increased solder contact area and extension ofthe solder joints into the recesses, may benefit the mechanicalreliability of the solder joints, and, in addition, may affect theelectrical performance of the connection. For example, the impedance ofthe connection may be changed by the increased solder volume. Thus, itmay be preferable that the solder joint is distributed along the lengthof a signal line in the direction of signal propagation. As such, anarrow, deep recess may be preferable to a wide, shallow recess. Theshape of the recess may be characterized, in part, by an aspect ratio ofdepth to width. In some embodiments, the aspect ratio of the recess maybe in a range of 1 to 4, but this is not a limitation.

As shown in FIG. 2 , ground leads 130 and 132 may be formed as part of ashield that encircles the signal leads 120 and 122.

A cross section of printed circuit board 110 in accordance withembodiments is shown in FIG. 3 . The printed circuit board 110 includesa plurality of layers including conductive layers 310, 312, . . . 320separated by dielectric layers 330, 332, . . . 340 in an alternatingarrangement. The printed circuit board 110 further includes a signal via350. FIG. 3 shows the layered structure of printed circuit board 110. Itwill be understood that an actual printed circuit board may includemultiple, closely spaced vias arranged in via patterns as describedbelow.

As shown in FIG. 3 , the printed circuit board 110 includes multiplelayers, each layer including a conductive layer and a dielectric layerso that the printed circuit board 110 includes an alternatingarrangement of conductive layers and dielectric layers. Each conductivelayer may serve as a ground plane, may be patterned to form conductivetraces, or may include a ground plane and conductive traces in differentareas. The layers may be formed, during assembly, by stacking multiplesheets of laminate with patterned copper and prepreg and then pressingthem under heat to fuse all the sheets. Patterning the copper may createtraces and other conductive structures within the printed circuit board.As a result of fusing, the layers may not be structurally separable in afinished printed circuit board. However, the layers may nonetheless berecognized in the fused structure based on the positions of theconductive structures. Accordingly, FIG. 3 illustrates separationbetween the layers of dielectric material, even though those layers maybe fused in a printed circuit board. The number of conductive layers anddielectric layers may vary according to application.

As further shown in FIG. 3 , the conductive layers 310, 312, . . . 320are removed in areas around signal via 350, except in a breakout layer352. For example, an antipad 344 is formed between conductive layer 310and signal via 350. The areas around signal via 350 where the conductivelayers are removed, known as antipads, provide clearance between signalvia 350 and those conductive layers which are not intended forconnection to signal via 350. In breakout layer 352, a signal trace 360is connected to signal via 350. The signal via 350 and the signal trace360 provide an electrical connection between a signal lead of surfacemount connector 100 (FIGS. 1 and 2 ) and another element of printedcircuit board 110.

As further shown in FIG. 3 , the signal via 350 is formed as aconductive element 362 that extends from top surface 364 of printedcircuit board 110 at least to breakout layer 352 where signal trace 360is connected to signal via 350. The signal via 350 may be formed bydrilling a hole through the conductive layers and the dielectric layersof printed circuit board 110 and filling the hole with a conductivematerial. The conductive element 362 may be generally cylindrical inshape.

The signal via 350 may be backdrilled from the rear surface of printedcircuit board 110 to a level slightly below breakout layer 352. Thebackdrilled portion of the hole is free of conductive material andavoids the formation of an electrical stub which may cause undesiredresonance in the transmission characteristic of the signal line.

The signal via 350 may further include a conductive pad 370 on the topsurface 364 of the printed circuit board 110. The conductive pad 370 maybe formed by patterning conductive layer 310 on top surface 364 and isin electrical contact with conductive element 362 of signal via 350. Theconductive pad 370 has a larger diameter than conductive element 362 andsurrounds conductive element 362 on top surface 364.

As shown in FIG. 3 , the signal via 350 includes a recess 380. Therecess 380 extends from a top surface 382 of signal via 350 into theprinted circuit board. The recess 380 may be characterized by a maximumdiameter, a maximum depth and a shape. In the embodiment of FIG. 3 , therecess 380 has a conical shape. However, this is not a limitation, andother shapes may be utilized, as discussed below.

By way of example only, the signal via 350 may have a diameter in arange of 0.10 mm to 0.30 mm and the conductive element 362 may extendall or part way through the printed circuit board 110. The recess 380may have a diameter in a range of 0.08 mm to 0.28 mm and a depth in arange of 0.05 mm to 0.30 mm. In some embodiments, recess 380 may have ashape that concentrates solder generally to match the shape of connectorlead 390. The diameter of the recess is based on the diameter of thesignal via 350 so as to maintain a minimum wall thickness of the signalvia 350 in the region of recess 380. The minimum wall thickness may bein a range of 0.02 mm to 0.08 mm. It will be understood that thesedimensions are given by way of example only and are not limiting.

A function of the recess 380 is to receive at least a tip portion of aconnector lead 390 of a surface mount connector. The tip portion of theconnector lead 390 of the surface mount connector may be tapered,beveled or otherwise configured, to enable insertion of the tip portioninto the recess 380. The dimensions and shape of the recess 380 may beselected to engage the tip portion of the connector lead of a particularsurface mount connector, and different recess sizes and shapes may beutilized with different surface mount connectors. It is not the intentof the recess 380 to accept the entire connector lead 390 of a surfacemount connector, as in the case of a contact tail of a press fitconnector which is inserted into a corresponding via of the printedcircuit board. Further, it is not the intent that the connector lead 390of the surface mount connector rest on the surface of the printedcircuit board without extending at least partially into the recess 380.By providing a configuration where the tip portion of the connector lead390 of the surface mount connector extends into the recess 380,advantageous features, including but not limited to mitigating theeffects of lack of coplanarity of the connector leads and improving thereliability of solder connections, may be achieved.

In some embodiments, the diameter and/or cross section of the connectorlead 390 may be larger than the diameter of the recess 380, except atthe tip portion of connector lead 390. In other embodiments, thediameter and/or cross section of the connector lead is smaller than thediameter of the recess 380, and insertion of the connector lead into therecess 380 is limited by the depth of the recess. In some cases, not allconnector leads may extend into the respective recesses due to lack ofcoplanarity of the tip portions of the connector leads. In someembodiments, the recess 380 and the tip portion of connector lead 390have different sizes and shapes, such that solder fills spaces betweenrecess 380 and connector lead 390 and increases the reliability of thesolder joint.

In some embodiments, the recess 380 may be centered with respect tosignal via 350. In other embodiments, the recess 380 may be offset withrespect to the center of signal via 350. The offset configuration may beused to control the impedance of the signal lines in a transition regionbetween the printed circuit board and the surface mount connector. Whenthe recess 380 is offset with respect to the center of signal via 350, aminimum wall thickness of the via is maintained in order to ensureelectrical continuity of the via. The recess 380 typically has acircular cross section in a plane parallel to the top surface of theprinted circuit board, but this is not a limitation.

A via 410 having a truncated conical recess 420 is shown in FIG. 4 . AV-shaped recess, such as is characteristic of a conical recess or atruncated conical recess, provides a gradual transition in impedancefrom the connector to the printed circuit board. Further, the V-shapedrecess provides self-centering of the connector lead and has a favorabledepth to volume ratio for forming a reliable solder joint.

A via 510 having a cylindrical recess 520 is shown in FIG. 5 . Acylindrical recess with a flat bottom provides a transition in impedancebetween the surface mount connector 100 and the printed circuit board110. A cylindrical recess can accept larger leads below the surface, ascompared with a conical or truncated conical recess, for enhanced solderjoint shear resistance. The cylindrical recess allows greater recessdepths to improve shear resistance without the wall thickness issuesassociated with conical and truncated conical recesses.

A via 610 having a recess 620 is shown in FIG. 6 . The recess 620 has ahemispherical portion 622 and a cylindrical portion 624. A recess with around-shaped bottom provides a transition in impedance between thesurface mount connector 100 and the printed circuit board 110. Thehemispherical recess can accept larger leads, as compared with theconical and truncated conical recesses, for enhanced solder joint shearresistance. The round-shaped bottom provides enhanced stress resistanceby eliminating sharp corners that may act as stress concentration pointsduring thermal cycling. In some embodiments, the cylindrical portion 624may be omitted, such that the recess 620 includes only hemisphericalportion 622.

The shape of the recess may depend in part on the process used to formthe recess. For example, when the recess is drilled, the shape of therecess corresponds to the shape of the drill bit. In other embodiments,the recess may be formed by laser drilling and have a shape thatconforms to laser drilling processes. Further, the size and shape of therecess may be selected based on the size and shape of the tip portion ofthe connector lead of the surface mount connector. The recess may haveany size and shape that is compatible with the drilling process andwhich is capable of receiving a tip portion of the connector lead of thesurface mount connector. Further, the shape of the recess may includeportions with different shapes, such as hemispherical portion 622 andcylindrical portion 624, as shown in FIG. 6 , a truncated conicalportion and a cylindrical portion (not shown), a conical portion and acylindrical portion (not shown), etc.

The depth of the recess is based on several factors. Where the bottom ofthe recess is not flat, the depth may be defined as the depth of therecess at its deepest point. The depth may be sufficient to compensatefor a lack of coplanarity of the tip portions of the connector leads ofthe surface mount connector. For example, the depth of the recess may bebased on the coplanarity specification of the surface mount connectorsuch that the longest connector leads extend to or near the bottom ofthe recess, whereas the shortest connector leads are located at or nearthe top of the recess.

In some embodiments, the depth of the recess may be selected based onthe tolerance with which the distal tip of connector lead 390 may bepositioned with respect to top surface 364 when a connector is mountedto the printed circuit board. That tolerance may depend on themanufacturing tolerances of the connector and/or the printed circuitboard. In some embodiments, that tolerance may be +/−0.08 mm. Theconnector and printed circuit board may be designed such that the tip ofconnector lead 390 is nominally 0.04 mm below the surface, such that, ifthe position of tip of lead 390 varies by up to 0.08 mm, it will neitherfail to fit fully within the recess nor fail to contact solder in therecess. With such a design, a high percentage of connector leads in aconnector footprint will extend, at least somewhat, into a correspondingrecess. That percentage may be greater than 95%, such as greater than98% or greater than 99%, in some embodiments.

Further, the depth of the recess may be based on the volume of solderrequired to achieve a reliable connection between the connector lead andthe via. For example, the shear strength of the solder joint may beenhanced by providing a recess having a greater depth. The depth of therecess is limited in the case of a conical or truncated conical shape,due to the requirement for maintaining a minimum wall thickness of thevia in the region of the recess. As shown in FIG. 4 , the wall thickness430 of truncated conical via 420 decreases as the depth of via 420 isincreased. In some embodiments, the depth of the recess may be in arange of 0.05 mm to 0.30 mm, but this is not a limitation.

A minimum wall thickness of the via in the region of the recess ismaintained to ensure electrical continuity of the via. As shown in FIG.5 , the via 510 has a wall thickness 530 in the region of cylindricalrecess 520. The depth of cylindrical recess 520 can be increased withoutdecreasing the wall thickness 530. However, in the case of truncatedconical recess 420 or conical recess 380, a wall thickness 430 decreasesas the depth of the recess is increased. Thus, the depth of a conical ortruncated conical recess is limited, depending on the required wallthickness and the angle of the conical portion of the recess. The wallthickness of the via in the region of the recess may be in a range of0.02 mm to 0.08 mm, but this is not a limitation.

A printed circuit board and surface mount connector leads, illustratingcompensation for lack of coplanarity of the connector leads inaccordance with embodiments, is illustrated in FIGS. 7A-7C. A printedcircuit board 710 is provided with vias 720, 722, 724 and 726, eachhaving a truncated conical recess 730. Connector leads 740, 742, 744 and746 of a surface mount connector engage the vias 720, 722, 724 and 726,respectively. Connector leads 740, 742, 744 and 746 lack coplanaritysuch that connector lead 740 has the shortest length, connector lead 746has the greatest length, and connector leads 742 and 744 haveintermediate lengths. Connector lead 746 is fully inserted into therecess of via 726, whereas connector lead 740 is positioned slightlyabove via 720.

Solder bricks 750 formed of solder paste are shown in FIG. 7B and solderjoints 760 are shown in FIG. 7C after heating of the solder bricks 750.As shown in FIG. 7C, the solder joints 760 connect each of the connectorleads 740, 742, 744 and 746 to the respective vias and provide reliableconnections. Even connector lead 740 with the shortest length isconnected to via 720 by a solder joint. Connector lead 740 is an extremeexample of a connector lead that is still connected to a via.

A prior art printed circuit board and surface mount connector leads areshown in FIGS. 8A-8C. A printed circuit board 810 includes vias 820,822, 824 and 826, each having a flat upper surface. Surface mountconnector leads 840, 842, 844 and 846 correspond to the vias 820, 822,824 and 826, respectively. The connector leads 840, 842, 844 and 846 mayhave flat bottom surfaces for improved contact with the respective vias.The illustrated configuration results from the connector leads, whichhave broadsides connected by edges, being bent such that the broad sidesare parallel to the printed circuit board 810. Connector leads 840, 842,844 and 846 lack coplanarity such that connector lead 840 has theshortest length, connector lead 846 has the greatest length, andconnector leads 842 and 844 have intermediate lengths. Solder bricks 850are shown in FIG. 8B, and solder joints 860 after heating of the solderbricks are shown in FIG. 8C. As shown in FIG. 8C, the connector lead 840may not make contact with the via 820 as a result of its shorter length,resulting in a defective solder joint.

Solder bricks are typically made up of 50% metal particles in the shapeof spheres and 50% flux and other volatiles. The volatiles evaporate bythe end of the thermal cycle, resulting in a metal volume that is onehalf of the original solder brick volume. The reduction in solder brickvolume presents a challenge to bridging the gaps caused by variation insurface mount lead length.

The printed circuit board having vias and/or other conductive elementswith recesses as described herein provides enhanced reliability ascompared with prior art printed circuit boards. In particular, therecesses compensate for lack of coplanarity of the tip portions of theconnector leads of the surface mount connector. In addition, therecesses permit increased solder volume, increased solder joint surfacearea and increased shear resistance as compared with prior art viashaving flat surfaces, and therefore the reliability of each solder jointis increased. In some embodiments, the coplanarity specification of theconnector leads of the surface mount connector may be relaxed when thevias are provided with recesses.

By eliminating the need for vias of sufficient size to receive contacttails of a press fit connector, the diameters of the vias in a regionnear the top surface of the printed circuit board can be reduced. Thereduced via diameters permit closer center-to-center spacing of vias andpermit routing of signal traces in the region near the top surface ofthe printed circuit board.

A top view of a printed circuit board having via patterns in accordancewith embodiments is shown in FIG. 9 . A printed circuit board 910includes via patterns 920, 922, 924 and 926. Each via pattern isconfigured for solder attachment to connector leads of a surface mountconnector as described herein. The printed circuit board may include anynumber of via patterns. The via patterns may be arranged in rows andcolumns, and the rows and/or columns may be aligned to form a grid ormay be offset. In some embodiments, multiple surface mount connectors100 may be held together with a housing or other support member asmodules of a larger connector. Those modules may be held in the housingin a pattern matching the pattern of via patterns shown in FIG. 9 , forexample.

As further shown in FIG. 9 , via pattern 920 may include signal vias 930and 932, which form a differential signal pair, and ground vias 940,942, 944 and 946. The via pattern 920 may further include shadow vias950, 952, 954 and 956 which interconnect ground planes of the printedcircuit board but are not connected to the surface mount connector. Inaddition, via pattern 920 may include an antipad 960, which is an areaaround signal vias 930 and 932 in which the conductive layer on the topsurface of printed circuit board 910 has been removed. It will beunderstood that the via patterns may have a variety of configurationsdepending at least in part on the configuration of the surface mountconnector and the operating frequency.

Each via pattern shown in FIG. 9 is configured for solder attachment toa surface mount connector (not shown in FIG. 9 ). The surface mountconnector may include signal leads that are attached to the respectivesignal vias 930 and 932 and ground leads that are attached to therespective ground vias 940, 942, 944 and 946. The signal vias 930 and932 and the ground vias 940, 942, 944 and 946 may be provided withrecesses as described herein so as to enable reliable solder attachmentbetween the surface mount connector and the printed circuit board 910.The shadow vias 950, 952, 954 and 956 are not attached to the surfacemount connector and may not be provided with recesses.

In one example, the signal vias 930 and 932 of via pattern 920 may havediameters of 0.25 mm and be spaced by 0.90 mm. The ground vias 940, 942,944 and 946 may have diameters of 0.20 mm and may extend through theprinted circuit board 910. The overall via pattern 920 may havedimensions of 2.0 mm or less and may be spaced from other via patternsby 2.0 mm or less. It will be understood that these dimensions are givenby way of example and are not limiting.

A printed circuit board configured for attachment to a conventionalpress fit connector may have a similar layout to the via patterns 920,922, 924 and 926 shown in FIG. 9 and described above. However, becausethe press fit connector relies upon contact tails that are inserted intorespective vias in the printed circuit board, the dimensions of a pressfit via pattern are larger than the dimensions of a surface mount viapattern as described herein. In particular, the signal vias and theground vias configured to accept contact tails of a press fit connectorare larger in diameter than corresponding vias configured for solderattachment to a surface mount connector, and the center-to-centerspacing between vias is larger to achieve a desired impedance and tolimit crosstalk between conductors. Accordingly, it is possible toachieve greater miniaturization using surface mount connectors and theprinted circuit boards described herein, as compared with press fitconnectors and corresponding printed circuit boards.

Referring again to FIG. 1 , a transition region 190 between surfacemount connector 100 and printed circuit board 110 includes a lowerregion of surface mount connector 100 adjacent to printed circuit board110 and an upper region of printed circuit board 110 adjacent to surfacemount connector 100. In the transition region 190, it is desirable tocontrol the impedances of the signal transmission lines to avoidimpedance variations and discontinuities. This may be difficult toachieve since the surface mount connector 100 and the printed circuitboard 110 have different structures and different materials. In someembodiments, impedance variations can be controlled by controlling thespacing and dimensions of the conductors in the connector 100 and in theprinted circuit board 110 and by controlling the dielectric constants ofthe materials of the connector 100 and the printed circuit board 110,for example.

In some embodiments, the connector leads of the surface mount connectormay be formed of superelastic materials. For example, the signal leads120 and 122 shown in FIGS. 1 and 2 may be formed of superelasticmaterials. Use of superelastic materials can enable the overall densityof the printed circuit board via patterns to be increased.

Having thus described several aspects of at least one embodiment, it isto be appreciated various alterations, modifications, and improvementswill readily occur to those skilled in the art. Such alterations,modifications, and improvements are intended to be part of thisdisclosure, and are intended to be within the spirit and scope of theinvention. Accordingly, the foregoing description and drawings are byway of example only.

What is claimed is:
 1. An interconnection system comprising: a surfacemount component comprising surface mount leads; and a printed circuitboard comprising conductive layers separated by dielectric layers, andvias configured for solder attachment to respective leads of the surfacemount component, each of the vias including a conductive element havinga recess in an upper surface thereof, wherein the recess in theconductive element is smaller in diameter than a surface mount lead ofthe surface mount leads and receives only a tip portion of the surfacemount lead.
 2. The interconnection system as defined in claim 1, whereintip portions of the surface mount leads are configured to be insertedinto respective recesses of the printed circuit board.
 3. Theinterconnection system as defined in claim 1, wherein the recessincludes at least one of a conical portion, a truncated conical portion,a cylindrical portion and a hemispherical portion.
 4. Theinterconnection system as defined in claim 1, wherein the surface mountconnector leads comprise superelastic components.
 5. The interconnectionsystem as defined in claim 1, wherein at least 95% of the surface mountleads comprise distal tips extending into respective recesses.
 6. Theinterconnection system as defined in claim 1, wherein the recess has adepth in a range of 0.05 mm to 0.30 mm.